Surface-coated sintered hard body

ABSTRACT

This invention relates to a surface-coated sintered hard body comprising an alloy consisting of at least one of carbides and carbonitrides of Group IVa, Va and VIa transition metals, cemented by at least one of metals and alloys, and two interior and exterior coated layers, the interior layer being a monolayer or multilayer consisting of at least one of carbides, carbonitrides and nitrides of Group IVa, Va and VIa transition metals, in which a part of the non-metallic element or elements are optionally replaced by oxygen, and at least one layer of the interior layer consisting of at least one of carbides, carbonitrides and nitrides of Group IVa, Va and VIa transition metals, in which a part or all of the non-metallic element or elements are replaced by boron, and the exterior layer consisting of at least one of aluminum oxide, zirconium oxide and mixtures or compounds thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a surface-coated sintered hard body and moreparticularly, it is concerned with surface-coated cemented carbide orcarbonitride alloys having at least one boron-containing coating layer,which exhibit good performances.

2. Description of the Prior Art

The so-called surface-coated cemented carbides or carbonitrides articleshaving a thin coating layer of TiC or TiN possessing excellent wearresistance have widely been put to practical use as a cutting toolhaving more excellent toughness as well as more excellent wearresistance than the prior art cemented carbides or carbonitridesarticles. These cemented carbides or carbonitrides articles consist ofat least one of carbides and carbonitrides of at least one elementselected from titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum and tungsten, cemented by at least one metal fromthe iron group. The TiC-coated cemented carbides or carbonitrides havemore excellent cutting properties than the prior art cemented carbidesor carbonitrides, but the cutting properties have hitherto been improvedonly to some extent because titanium carbide meets with a markedlowering of hardness at a high temperature and lacks in oxidationresistance. In order to overcome these disadvantages, cemented carbidealloys coated with ceramic materials such as Al₂ O₃ and ZrO₂ have beenproposed. However, the adhesive strength of such a ceramic layer to acemented carbide alloy is not sufficient for practical use and,therefore, it has been proposed to use at least one of carbides andnitrides of metals such as titanium, zirconium, etc. between the ceramiclayer and cemented carbide alloy. In fact, this laminated article hasreally been used. When, for example, a titanium carbide and/or nitrideis used as a layer under an Al₂ O₃ layer as described above, however,the low oxidation resistance at a high temperature, which is one of theabove described two disadvantages of the TiC-coated cemented carbides,is somewhat improved, but the lowering of the hardness at a hightemperature as a whole is only improved partly. That is to say, even ifthe surface is coated with a ceramic material whose hardness is not solowered even at a high temperature, such lowering of hardness of thecoated layers as a whole is unavoidable as far as TiC is used under theceramic layer.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface-coatedsintered hard body having excellent properties.

It is another object of the present invention to provide cementedcarbide or carbonitride alloy coated with a boron-containing layer,which are excellent in hardness at a high temperature.

It is a further object of the present invention to provide asurface-coated sintered hard body having, as an intermediate layer alayer, having a sufficient adhesive strength to both cemented carbidesor carbonitrides and ceramic materials.

These objects can be attained by a surface-coated cemented carbides orcarbonitrides article having at least one layer consisting of at leastone material selected from boronitrides and borocarbonitrides of GroupIVa, Va and VIa elements.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a pseudo-binary phase diagram of TiN--TiB₂at 1000° C.

DETAILED DESCRIPTION OF THE INVENTION

We, the inventors, have reached the present invention by choosing amaterial whose hardness is not so lowered even at a high temperature andwhich has a sufficient adhesive strength to a cemented carbide orcarbonitride and a ceramic material such as alumina or zirconia, as anintermediate layer between them. We have already found that whencemented carbide or carbonitride articles are coated with carbides,nitrides and oxides of Group IVa, Va and VIa transition metals, andmixtures or compounds thereof, by the chemical vapor deposition method,the adhesive strength between the coated layer and cemented carbide orcarbonitride substrate is decreased in the order of the carbides,nitrides and oxides, and it is practically preferable to use thecarbides or carbonitrides. Further, we have now found that carbides,nitrides and oxides of Group IVa, Va and VIa transition metals, andmixtures or compounds thereof, in which a part or all of thenon-metallic constitutional element or elements are replaced by boron,have a hardness that is not so lowered even at a high temperature. Thepresent invention is based on this finding.

In accordance with the present invention, therefore, there is provided asurface-coated sintered hard body comprising an alloy consisting of atleast one of carbides and carbonitrides of Group IVa, Va and VIatransition metals, in which a part of the non-metallic element orelements can be replaced by oxygen and boron, cemented by at least onemember selected from the group consisting of the iron group metals,tungsten, chromium and molybdenum and alloys thereof, and two interiorand exterior coated layers, the interior layer being a monolayer ormultilayer consisting of at least one of carbides, carbonitrides andnitrides of Group IVa, Va and VIa transition metals, in which a part ofthe non-metallic element or elements are replaced optionally by oxygen,and at least one layer of the interior layer consisting of at least oneof carbides, carbonitrides and nitrides of Group IVa, Va and VIatransition metals, in which a part (in the case of a monolayer) or allof the non-metallic element or elements are replaced by boron, and theexterior layer consisting of at least one of aluminum oxide (Al₂ O₃) andzirconium oxide (ZrO₂) and mixtures or compounds thereof including solidsolutions.

The binder metal in the above mentioned sintered hard body can furthercontain the hard phase-forming elements and ordinary additives such asaluminum, silver, copper, phosphorus, silicon, etc.

In the present invention, Group IVa metals of the Periodic Table meantitanium (Ti), zirconium (Zr) and hafnium (Hf), Group Va metals meanvanadium (V), niobium (Nb) and tantalum (Ta) and Group VIa metals meanchromium (Cr), molybdenum (Mo) and tungsten (W). Of the interior layers,the most interior layer in contact with the surface of the cementedcarbide or carbonitride alloy consists preferably of carbides and/orcarbonitrides of Group IVa, Va and VIa transition metals and the outsideinterior layer in contact with the exterior layer consists preferably ofthe hard compounds in which a part of the non-metallic elements arereplaced by oxygen. Even if the interior layer is not of a clearlaminate structure, but has a composition which changes gradually orcontinuously, the effects of the present invention can similarly beobtained.

In the present invention, the thickness of the coating layers, as awhole, is preferably 2 to 10 microns, that of the interior layer beingpreferably 0.1 to 10 microns and that of the exterior layer being 0.1 to5 microns.

One of the important properties required for such a coating material isa high hardness at such a high temperature to which a tool edge isexposed during a cutting operation, for example, at about 1000° C. Fromthis point of view, diborides of Ti, Zr and Hf, in particular, TiB₂,having a remarkably high hardness at a high temperature are mostpreferable, but, in fact, these diborides are not so preferable becauseof their reaction with iron as a workpiece to be cut, resulting in amarked progress of crater wear. The inventors have made various effortsto suppress this reaction with iron and have noted that nitrides of Ti,Zr and Hf, in particular, TiN, lack in reactivity with iron and show avery small crater wear. However, TiN has only a low hardness at a hightemperature and it is thus assumed that a substance having anintermediate property between those of TiN and TiB₂ will satisfy boththe hardness at a high temperature and the reactivity with iron.

In one aspect of the present invention, therefore, there is provided asurface-coated cemented carbide or carbonitride article having at leastone coating layer consisting of at least one material selected fromboronitrides and borocarbonitrides of Group IVa, Va and VIa elements.Particularly, one preferred embodiment of the present invention consistsin a cemented carbide or carbonitride alloy surface-coated with onelayer consisting essentially of a mixture of 0 to 50%, preferably 5 to50%, by weight of at least one of diborides of Ti, Zr and Hf and 50 to100%, preferably 50 to 95%, by weight of at least one of boronitridesand borocarbonitrides of Ti, Zr and Hf. If necessary, this coating layercan further be coated with a ceramic material such as Al₂ O₃ as anexterior layer.

The inventors have made various samples by the chemical vapor depositionmethod to study the phase diagram of TiN and TiB₂ and, consequently,have found that there is a two phase coexistent zone of Ti(BN) and TiB₂and a one phase zone of Ti(BN) at about 1000° C. as shown schematicallyin the accompanying drawing. A sample of a surface-coated articleprepared based on this result has an advantage as expected. As to thequantity of TiB₂, it should not exceed 50% by weight since if more than50% by weight, the reaction with iron as a workpiece cannot beneglected. As to Ti(BN), a part of the non-metallic elements can bereplaced by carbon and yet still obtain the substantial effect of thepresent invention.

Although the above described illustration is limited to Ti only, thepresent invention can of course be applied to Zr and Hf with similaradvantages. In addition, the advantages or merits of the presentinvention are scarcely affected by the presence of small amounts ofimpurities and additives.

The above described boron-containing coating layer can be used with oneor more other coating layers to thus give synergistic effects. When theboron-containing coating layer is further coated with, for example,aluminum oxide, which is excellent in oxidation resistance, theoxidation resistance of the surface-coated article as a cutting tool canmarkedly be improved.

The coating layer of the present invention can best be formed by thechemical vapor deposition method, but physical vapor deposition methodssuch as ionic plating, sputtering, vacuum vapor deposition and the like,and metallizing plating methods such as plasma spraying, flame sprayingand the like can also be applied.

The present invention will be further illustrated in greater detail inthe following examples. It will be self-evident to those skilled in theart that the ratios and ingredients in the following formulation and theorder of operations can be modified wihtin the scope of the presentinvention. Therefore, the present invention is not to be interpreted asbeing limited to the following examples. All percents are to be taken asthose by weight unless otherwise indicated.

EXAMPLE 1

A cemented carbide alloy of ISO P 30 (free carbon precipitated: 0.03%;Form No.: SNU 432) was heated in a mixed stream of TiCl₄, CH₄, H₂ and N₂in a reactor of Inconel (Commercial Name, nickel alloy manufactured byInco Co.) to deposit a coating of Ti(CN), and thereafter, there wasdeposited Ti(BN) (B-substituent of TiN) from a mixed stream of TiCl₄,BCl₃, H₂ and N₂ in the same reactor. Then, there was further depositedTi(CO) (O-substituent of TiC) from a mixed stream of TiCl₄, CH₄, CO₂, COand H₂ in the same reactor. The thickness of the interior layersamounted to 6 microns. After cooling, the thus surface-coated cementedcarbide alloy was further heated in a mixed stream of AlCl₃, CO₂ and H₂in a reactor of sintered alumina to thus precipitate a coating of Al₂ O₃with a thickness of 1 micron. An insert obtained in this way willhereinafter be referred to as Insert Sample A.

Various Insert Samples B to L were prepared in an analogous manner tothat described above and, for comparison, a commercially sold TiC-coatedinsert (Insert Sample M) and an Al₂ O₃ /TiC coated insert (Insert SampleN) were prepared, which were then subjected to a cutting test under thefollowing conditions:

    ______________________________________                                        Workpiece to be cut  : FC 30                                                  Cutting Speed        : 150 m/min                                              Feed                 : 0.40 mm/rev                                            Cutting Depth        : 2 mm                                                   No Cutting Aid                                                                ______________________________________                                    

The results are shown in Table 1:

                  TABLE 1                                                         ______________________________________                                        Insert                                                                              Exterior  Interior Layers (in order of from                                                                  Life                                     Sample                                                                              Layer     outside)             (min)                                    ______________________________________                                        A     Al.sub.2 O.sub.3                                                                      1μ Ti(CO)--Ti(BN)--Ti(CN)                                                                         6μ                                                                             49                                   B     ZrO.sub.2                                                                             1μ Ti(CO)--Ti(BN)--Ti(CN)                                                                         6μ                                                                             36                                   C     Al.sub.2 O.sub.3                                                                      1μ Ti(CO)--Ti(BCN)--Ti(CN)                                                                        6μ                                                                             41                                   D     Al.sub.2 O.sub.3                                                                      1μ Ti(CO)--Ti(BC)--TiC                                                                            6μ                                                                             33                                   E     Al.sub.2 O.sub.3                                                                      1μ Ti(BN)--Ti(CN)   6μ                                                                             39                                   F     Al.sub.2 O.sub.3                                                                      1μ Ti(CO)--Ti(BCO)--Ti(CN)                                                                        6μ                                                                             37                                   G     Al.sub.2 O.sub.3                                                                      1μ Nb(CO)--Nb(BN)--Nb(CN)                                                                         6μ                                                                             46                                   H     Al.sub.2 O.sub.3                                                                      1μ Nb(CO)--Nb(BCN)--Nb(CN)                                                                        6μ                                                                             40                                   I     Al.sub.2 O.sub.3                                                                      1μ Zr(CO)--Zr(BN)--Zr(CN)                                                                         6μ                                                                             38                                   J     Al.sub.2 O.sub.3                                                                      1μ Hf(CO)--Hf(BN)--Hf(CN)                                                                         6μ                                                                             47                                   K     Al.sub.2 O.sub.3                                                                      1μ Ta(CO)--Ta(BN)--Ta(CN)                                                                         6μ                                                                             36                                   L     Al.sub.2 O.sub.3                                                                      1μ Ta(CO)--Ta(BN)--Ta(CN)                                                                         6μ                                                                             37                                   M      --           TiC              7μ                                                                              9                                                                            Crater                                                                        Wear                                 N     Al.sub.2 O.sub.3                                                                      1μ TiC              6μ                                                                             31                                                                            Flank                                                                         Wear                                 ______________________________________                                    

EXAMPLE 2

A cemented carbide alloy of ISO P-30 (Form No. SNU 432) was heated andheld at 1000° C. in a mixed stream of BCl₃, TiCl₄, N₂ and H₂. Thecoating thickness was 7μ. Analysis of the surface by X-ray diffractionafter cooling showed that there were two phases of 30% by weight of TiB₂and 70% by weight of Ti(BN). The resulting insert sample and acommercially sold TiC-coated insert sample were subjected to a cuttingtest under the following conditions:

    ______________________________________                                        Workpiece to be cut                                                                              : SCM 3 (H.sub.B = 280)                                    Cutting Speed      : 170 m/min                                                Feed               : 0.36 mm/rev                                              Cutting Depth      : 2 mm                                                     Cutting Time       : 40 minutes                                               ______________________________________                                    

Thus, the insert of the present invention showed a crater wear of 0.04mm and a flank wear of 0.18 mm, while the commercially sold TiC-coatedinsert showed a crater wear of 0.14 mm and a flank wear of 0.36 mm.

EXAMPLE 3

The same cemented carbide alloy as that of Example 2 was heated and heldat 1000° C. in a mixed stream of CH₄, BCl₃, TiCl₄, N₂ and H₂. Thecoating thickness was 7μ. Analysis of the surface in an analogous mannerto Example 2 after cooling showed that there were two mixed phases of30% by weight of TiB₂ and 70% by weight of Ti(BNC). When the resultinginsert was subjected to a cutting test under the same conditions asthose of Example 2, it showed a crater wear of 0.10 mm and flank wear of0.17 mm.

EXAMPLE 4

The same cemented carbide alloy as that of Example 2 was coated withvarious materials in an analogous manner to Example 2 and then subjectedto a cutting test under the same conditions as those of Example 2, inwhich the lives were measured by a period of time when the crater wearreached 0.15 mm or the flank wear reached 0.40 mm. The compositions ofthe coated layers and the test results are shown in Table 2:

                  TABLE 2                                                         ______________________________________                                        Insert                     Life                                               Sample  Composition of Coating Layer                                                                     (min)                                              ______________________________________                                        I       100% Ti(BN)     7μ  51                                             II      20% TiB.sub.2 --80% Ti(BN)                                                                    7μ  55                                             III     40% TiB.sub.2 --60% Ti(BN)                                                                    7μ  58                                             IV      60% TiB.sub.2 --40% Ti(BN)                                                                    7μ  31                                             V       80% TiB.sub.2 --20% Ti(BN)                                                                    7μ   8                                             VI      100% TiB.sub.2  7μ  more than 1                                    VII     100% TiC        7μ  37                                             ______________________________________                                    

EXAMPLE 5

The same cemented carbide alloy as that of Example 2 was coated withvarious materials in an analogous manner to Example 3 and then subjectedto a cutting test under the same conditions as those of Example 2, inwhich the lives were measured by a period of time when the crater wearreached 0.15 mm or the flank wear reached 0.40 mm. The compositions ofthe coated layers and the test results are shown in Table 3:

                  TABLE 3                                                         ______________________________________                                        Insert                     Life                                               Sample  Composition of Coating Layer                                                                     (min)                                              ______________________________________                                        I       100% Ti(BNC)     7μ 48                                             II      80% Ti(BNC)--20% TiB.sub.2                                                                     7μ 51                                             III     60% Ti(BNC)--40% TiB.sub.2                                                                     7μ 55                                             IV      40% Ti(BNC)--60% TiB.sub.2                                                                     7μ 12                                             V       20% Ti(BNC)--80% TiB.sub.2                                                                     7μ  3                                             VI      100% TiB.sub.2   7μ more than 1                                    VII     100% TiC         7μ 37                                             ______________________________________                                    

EXAMPLE 6

The 30% TiB₂ -70% Ti(BN)-coated cemented carbide alloy of Example 2 washeated and held at 850° C. in a mixed stream of AlCl₃, H₂ and CO₂ toform a coating of aluminum oxide (1μ). The thus coated insert, theinsert of Example 2 and the commercially sold TiC-coated insert werethen subjected to a cutting test under the following conditions:

    ______________________________________                                        Workpiece to be cut  : FC 25                                                  Cutting Speed        : 120 m/min                                              Cutting Depth        : 2 mm                                                   Feed                 : 0.65 mm/rev                                            No Cutting Aid                                                                ______________________________________                                    

The insert of this example could be used in the cutting operation for 41minutes while the insert of Example 2 was used for only 9 minutes andthe TiC-coated insert was used for only 4 minutes due to crater wear.

EXAMPLE 7

The same cemented carbide alloy as that of Example 2 was coated withvarious materials in analogous manners to Example 2 and Example 3 andthen coated with aluminum oxide in the same manner as Example 5. Thethus coated inserts and the commercially sold Al₂ O₃ /TiC coated insertswere then subjected to a cutting test under the same conditions as thoseof Example 6, in which the lives were measured by a period of time whenthe flank wear reached 0.40 mm or the crater wear reached 0.20 mm. Thecompositions of the coated layers and the test results are shown inTable 4:

                  TABLE 4                                                         ______________________________________                                        Insert Exterior                     Life                                      Sample Layer     Interior Layer     (min)                                     ______________________________________                                        I      Al.sub.2 O.sub.3                                                                      1μ 100% TiB.sub.2 6μ                                                                              9                                    II     Al.sub.2 O.sub.3                                                                      1μ 30% TiB.sub.2 --70% Ti(BNC)                                                                  6μ                                                                             38                                    III    Al.sub.2 O.sub.3                                                                      1μ 100% Ti(BNC)   6μ                                                                             31                                    IV     Al.sub.2 O.sub.3                                                                      1μ 70% TiB.sub.2 --30% Ti(BN)                                                                   6μ                                                                             11                                    V      Al.sub.2 O.sub.3                                                                      1μ 30% TiB.sub.2 --70% Ti(BN)                                                                   6μ                                                                             41                                    VI     Al.sub.2 O.sub.3                                                                      1μ 100% Ti(BN)    6μ                                                                             32                                    VII    Al.sub.2 O.sub.3                                                                      1μ TiC            6μ                                                                             30                                    ______________________________________                                    

What is claimed is:
 1. A surface-coated sintered hard body, whichcomprises a sintered hard alloy and at least one coating layer thereoncomprising at least one material selected from boronitrides andborocarbonitrides of Group IVa, Va and VIa elements.
 2. Thesurface-coated sintered hard body as claimed in claim 1, wherein thesintered hard alloy comprises at least one of carbides and carbonitridesof Group IVa, Va and VIa transition metals, cemented by at least onemember selected from the group consisting of the iron group metals,tungsten, chromium and molybdenum, and alloys thereof.
 3. Thesurface-coated sintered hard body as claimed in claim 1, wherein thecoating layer comprises a mixture of 5 to 50% by weight of at least oneof diborides of titanium, zirconium and hafnium and 50 to 95% by weightof at least one of boronitrides and borocarbonitrides of titanium,zirconium and hafnium.
 4. The surface-coated sintered hard body asclaimed in claim 3, wherein the coating layer is further coated with aceramic material selected from aluminum oxide, zirconium oxide andmixtures or compounds thereof.
 5. The surface-coated sintered hard bodyas claimed in claim 1, wherein the coating layer comprises interior andexterior layers, the interior layer being a monolayer or multilayercomprises at least one of carbides, carbonitrides and nitrides of GroupIVa, Va and VIa elements, in which a part of the non-metallic element orelements are optionally replaced by oxygen, and at least one layer ofthe interior layer comprises at least one of carbides, carbonitrides andnitrides of Group IVa, Va and VIa elements, in which a part or all ofthe non-metallic element or elements are replaced by boron, and theexterior layer comprising at least one of aluminum oxide, zirconiumoxide and mixtures or compounds thereof.
 6. The surface-coated sinteredhard body as claimed in claim 5, wherein the coating layers, as a whole,have a thickness of 2 to 10 microns, the interior layer having athickness of 0.1 to 10 microns and the exterior layer having a thicknessof 0.1 to 5 microns.
 7. The surface-coated sintered hard body as claimedin claim 5, wherein the coating layers are formed by chemical vapordeposition method.